Protecting the state’s population from West Nile virus

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Farm Forum

When West Nile virus hit the state in 2002, we were scrambling to find answers. Now we’re to the point where science and public policy are coming together. An increasing number of communities have established mosquito control programs, so the next step is to help them plan their control strategies to specifically target the mosquitoes that carry the virus.

My cohort Michael Wimberly, a senior scientist at the Geospatial Sciences Center of Excellence, uses earth-imaging data to identify environmental conditions that are conducive to amplification of the virus. I’m the mosquito guy and have been working with the South Dakota Department of Health since 2001 to help monitor the state’s mosquito population, particularly those infected with West Nile virus.

Here is some important information that has come from this work.

1. The mosquitoes that come out in early June tend NOT to be the ones that transmit West Nile Virus to humans. The nuisance mosquitoes, Aedes vexans, are the early season mosquitoes—they do not carry West Nile virus. However, from a public health perspective, the nuisance mosquitoes are our “friends” because their presence motivates us to use insect repellents.

2. Aedes vexans overwinter in the egg stage, which allows the population to explode quickly in the spring amid warming temperatures and spring rains. The West Nile virus-carrying Culex tarsalis female adults survive South Dakota winters, presumably in animal burrows. Consequently, it takes longer for the population to increase, typically at the end of June and early July. Also, in early summer, Culex tarsalis mosquitoes prefer to feed on birds rather than mammals.

3. West Nile virus is essentially a bird virus that also causes problems in humans and horses. Whether the virus itself overwinters in birds or mosquitoes has not yet been determined. However, the virus takes several weeks to increase enough to provide a substantial health risk to humans.

4. Thus far, warmer than normal winters and early springs have been associated with an increased risk for West Nile virus. These warmer temperatures may allow more Culex tarsalis females to survive winter.

5. Lack of rainfall doesn’t mean that the risk of West Nile virus is decreased. The West Nile mosquito breeds in small water puddles, such as wheel ruts in a pasture or irrigated alfalfa. Wimberly calls it the “Goldilocks effect–the water needs to be just right.” Consequently, hot summer temperatures have a greater impact on human transmission than precipitation.

6. In South Dakota the peak period for transmission of West Nile virus to humans extends from mid-July through the end of August, with Culex tarsalis females tending to feed later at night than the nuisance mosquito —usually between 10 p.m. and 2 a.m.

Consequently, communities must plan ahead when it comes to mosquito control. If the risk of West Nile virus is low, officials can allocate more of their resources to controlling the nuisance mosquitoes in June, but if the risk is high, they need to hold back so they can spray later in the season to control the West Nile virus-carrying mosquitoes, particularly during community events that extend into the last evenings.